Trading Partner Agreements/EDI

New

We are a billing service which bills for numerous providers and organizations in different states. Most of our billing will be done electroncially but in order to set up our EDI we need to obtain Trading Partner Agreements. My question to the forum; Would anyone have any suggestions as to where I would begin to look or who I would contact? Not all payor web-sites give you this information in an obtainable format. Thank you.